pwm: lpc-18xx: use pwm_set_chip_data

The lpc-18xx driver currently manipulates the pwm_device struct directly
rather than using the pwm_set_chip_data() function. While the current
method may save a clock cycle or two, using the explicit function call
makes it more obvious that data is set to the local chip data pointer.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Reviewed-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>